Perhaps the single most important question concerning World War I was whether it could have been avoided. Did the war occur only because of misunderstandings, errors and miscalculation? Or were there deep forces that made the war inevitable, regardless of best intentions, the desire to collaborate, and an understanding of what this war would mean? In the end, this is not just a question about this war, nor about war itself. It is a question of how human history is shaped. But as with all things philosophical, it is best to begin with a limited question, and examining the origins of World War I is a perfect starting point.

It is more than a test case. World War I initiated a period that was catastrophic to Europe and transformed the world. The 31 years from 1914 to 1945 saw the death of approximately 100 million Europeans from political causes—ranging from wars and genocide, to planned starvation and civil strife. It was one of the most barbaric periods humanity has ever seen. That it took place in a region that regarded itself as the most civilized in the world—the heir to the Enlightenment—made the slaughter even more extraordinary, and an explanation even more important.

The Europeans, after all, had discovered the concept of humanity—not only in thought, but in practice. European imperialism had shattered the sequestered peoples of the world. The Mongols had not known of the Aztecs, the Japanese had not known of the Zulus. There was no universal understanding of the extent and variations of human beings until the Europeans forced the world to know itself through Europe’s conquest and domination. Europe’s gift came with a terrible price, but that understanding, the knowledge of the educated Javanese that there were Eskimos, was nonetheless a gift. At the end of that 31 years, beginning in August 1914, that empire was shattered. Europe’s descent was not unprecedented, but the breadth of its power and the speed of the loss of this power was.

Did World War I have to happen, and did it have to take the shape it did, are urgent questions in another sense. After World War II, continental Europe, particularly the European peninsula, lost its sovereignty. The eastern part was occupied by the Soviet Union, the western by the United States. Each treated their regions very differently, but questions of war and peace depended on decisions made in Washington and Moscow—not in London, Paris or Berlin. And the question of war and peace is the essence of sovereignty. After the collapse of the Soviet Union, the Russians withdrew, the Americans became indifferent, and Europe was free to create its own future. It is interesting that the end of the Soviet Union in 1992 was the 500th anniversary of Columbus’s maiden voyage, and coincided with the implementation of the Maastricht Treaty.

What had begun 500 years before, ended in 1992. There was no longer a single global power in Europe. Europe had proceeded from being a marginal region to the center of gravity of the international system, and back to marginality. This was also an opportunity for Europe to create a new system, a new definition of power, a new relationship between nations. Perhaps the most important political question in the world today is whether that experiment can long endure, and that question also brings us back to World War I. If the question of World War I is whether it was impossible for political leaders to avoid it, then the importance of leaders, the policies they pursue, and the things they wish for, becomes mere sentiment. If they did not have control over events in the past, then perhaps European policy-makers don’t have control over events now. And if that is true, then perhaps the confidence Europe has that it had buried its demons is misplaced.

To understand World War I, it is important to bear in mind that Europe has persistently had systematic wars in which the leading powers engaged in battles that reshaped the Old Continent. The Seven Years’ Wars in the eighteenth century and the Napoleonic wars of the nineteenth all involved leading European powers; they were European wars with global implications, that reshaped the power structure of Europe. The question was not whether the twentieth century would have such wars, but what would cause them.

The cause was the unification of Germany. Before unification, the Germanic region was a weak and fragmented buffer between the Russian Empire and France. With the Alps separating northern and southern Europe, the north German plain, which had few natural barriers, had the Germanic states as a human and political barrier. It did not preclude war on the north European plain, but it focused it into two distinct regions, around the North Sea and the Baltics. It was as if there were two northern Europes.

The Napoleonic wars put in motion the process of German unification. The ascendance of Prussia after 1815 made it inevitable. The German nation-state was born in battle, forged in the Franco-Prussian War. Germany was an extraordinarily dynamic country, rapidly surpassing France in most economic categories and by 1913 equaling the United Kingdom economically—in spite of the fact that Britain was drawing on the protected resources of the British Empire, while Germany was competing for European markets and resources. The growth of the German economy, coupled with its substantial military power, staggered the European political system, relatively stable since 1815.

Germany was frightening to the United Kingdom, because it threatened to construct a formidable fleet, and given its industrial power, that could challenge Britain’s command of the sea, and therefore its empire. It frightened France, which remembered the defeat Germany had dealt it in the Franco-Prussian war, and understood that Germany had become even more powerful relative to France since then. It frightened Russia because it had discovered its core weakness in the Napoleonic wars, that massed artillery can rout an army, and that industrial powers had masses of artillery. France had been defeated by space—its lines of supply could not extend to Moscow. Germany was much closer than France was, its supply lines considerably shorter and its ability to manufacture masses of weapons staggering. It could move east from Prussia in a single assault, or systematic extension of its military and economic power. Germany fully understood their fears. They were reasonable and impossible to assuage. Germany was a massive power. It could not persuade them that it had no aggressive intent, because intent changes. Intent is merely words, and promises, and even if sincere, words carry no inherent weight and no nation can build its hopes on another’s intent. What was needed were guarantees that could not be violated, and the vast northern European plain did not allow for guarantees. It allowed only promises based on intent.

For Germany, the fear was a simultaneous attack by France and Russia, from the west and east, coupled with a British blockade. If Germany were attacked in this way, at a time and place of its potential enemy’s choosing, particularly if taken by surprise, it would be defeated, fragmented and returned to the status quo ante of 1871. And any reasonable analysis of the strategic situation showed that France, Russian and Britain had every reason to be terrified of Germany. The Triple Entente was formed out of shared fear, and that fear terrified Germany. The Anglo-French-Russian alliance did not emerge out of misunderstanding. It arose out of a clear understanding of German power. And the German fear did not arise from a misunderstanding. It arose from a clear understanding of the power of its enemies.

Germany understood that if it allowed its enemies to choose the time and place for war, Germany would lose. The hope that they would choose not to go to war had no historical or strategic basis. Germany had every reason to think that its growing power would force them to go to war. It therefore had to develop its own war plan, and it had to be one that had two principles. The first was that it had to be initiated by Germany at a time and place of its choosing, and that it had to be sequential, not simultaneous. It could not fight everyone at the same time. Therefore Germany had to go to war exercising enormous discipline and control of the battlefield.

What emerged was the Schlieffen Plan, a war plan that predated the Triple Entente, and that was based on the principle that whatever the current intent, such an alliance was already embedded in the geopolitical reality. It was thus a war plan—one that grew out of the national reality of Germany. It was too powerful not to frighten its enemies, and too weak not to defeat them all at once.

The plan focused its first blow on the French. Moving through Belgium and sweeping in an arc along the coastal plan along the English Channel, it massed an army to sweep around Paris and defeat it with such speed that the British had no time to intervene, or create a naval blockade that would succeed. With French railways and Atlantic ports in German hands, the simple North Sea blockade that would strangle Germany could not succeed. For any hope of success, the British would have to split the fleet, giving the German fleet a reasonable chance to defeat the British in the North Sea, or the British would have to simply cede the war to Germany.

Extraordinarily, the Germans grasped the logic of this position and were prepared to allow the Russians to attack from the east, and advance into Germany, even taking East Prussia, while the bulk of German forces were dealing with the French. They knew that if they broke the French, they could rapidly redeploy their forces to the Russian front, engage and defeat the Russian Army. In the meantime its Austro-Hungarian ally and Turkey, both relatively weak military forces, would distract the Russians.

The Schlieffen Plan existed for decades without use, continually updated. The Triple alliance existed without implementing any allied offensive action. But neither side doubted that at some point war would come, and each side had a war plan they intended to implement. But it was not the existence of the war plans or the growing armaments that constituted the threat to peace. It was the fact that human beings do not know what other human beings intend, nor do they always know what they themselves intend. At the heart of this entire tale is the single thing that all of us know and which shapes our relations with the world. Intentions are real, but changeable. Power is real, and takes much longer to change. At some point, the intention of someone would change and the sum of all fears would emerge.

The killing of Archduke Franz Ferdinand by a Serbian nationalist did not cause World War I. It simply escalated everyone’s fears to the point at which they had to act. Gavrilo Princip announced that the time had come. If not for Princip, there would have been some other cause. The decision of the Austro-Hungarians to mobilize led the Russians to mobilize and opened the door for the German nightmare, but at some point someone would have mobilized, and the fears of all those living on the north German plain would have been announced.

The Russian mobilization was slow. It had to be, given the nature of Russia. The French were faster but waited until the Germans began. This gave Germany the advantage, one it had planned on: let the Russians mobilize slowly. The Germans needed to beat the French. They did, but not by enough. Some argue that the mistake was that the German command lost its nerve at the last moment and failed to put everything into the right flank swinging around Paris. Some say that the French surprised everyone by innovating transport to the battlefield on the fly. Others say that the logistical challenge was too much for the Germans. In any case, the French stopped the Germans on the Marne River, preventing Paris’s envelopment. Even more surprising in its way, Russia did not take advantage of the opening provided by the Germans and were defeated at Tannenburg, never advancing into East Prussia.

The initiation of war went as expected. The war itself went as no one expected. Rather than the rapid defeat by one side or the other through the speed provided by rails and other transport and rapid breakthroughs by massed artillery smashing through fortifications and infantry, the war bogged down in a horrible and increasingly pointless slaughter.

The surprise is not that World War I happened. The surprise is how it was fought. And the surprise was that no one stopped it. Plans are intentions and the intention of the Schlieffen Plan was to take Paris with stunning speed. It didn’t happen. One of the most important reasons was the machine gun. The massed assaults of infantry that Napoleon had mastered, and that had dominated military thinking for a century, were impractical because of that simple innovation: the machine gun. The failure to understand the significance of new technology is frequently the Achilles’ heel of strategy. It leads intentions into the realm of wishful thinking.

The machine gun created a stalemate. Neither side could advance significantly, but neither side could stop trying. The mystery of World War I is not why it happened, nor why the war plans failed. Again, the mystery of the war was why it wasn’t stopped when it became clear that the machine gun made victory impossible—or that it made victory possible only at the cost of extraordinary destruction.

The answer in part is that everyone thought that with one more push they must be able to win. But surely by 1916 they knew better. The other partial answer is that the political situation at home meant that anything less than victory for the blood that was shed would delegitimize governments. To give your son for victory is one thing. Even to give him for defeat is understandable. But to give him and then simply stop and call it a mistake—that would be unbearable.

But I think the reason is simpler. It couldn’t stop because none of the reasons that it was fought in the first place had gone away. Everyone knew that it would start again. Indeed, after the Americans came and broke the stalemate, and the Versailles Treaty was signed, few believed that the war wouldn’t be replayed. And it was, with minor variations on a theme. The geopolitical realities were such that whatever our wishes and intents, that geopolitical crisis had to be fought out to its monstrous conclusion.

Some 600,000 casualties occurred in one week at the Battle of the Somme. The machine gun changed the European soul. It made the unthinkable not only possible but unremarkable. Those who died were gone; these who lived, simply lost their souls and their minds. After World War I began, Stalin’s purges, Hitler’s Holocaust, the Russian and Spanish civil wars all became conceivable. There was nothing that couldn’t be conceived anymore.

The geopolitical problem was solved in May 1945, when Germany was again occupied and partitioned, when France and Britain lost their empires (they took a while to realize it) and when sovereignty was taken away from Europe. And then a new geopolitical reality emerged—the Cold War. Everyone thought that the Cold War would lead to nuclear holocaust. The expectation was that the horror that began in 1914 would endlessly escalate until all of humanity was annihilated. Given what had happened between 1914 and 1945, what could have been more reasonable? All limits were gone, and Europe had turned monstrous.

But it turned out differently. The Americans and Soviets turned out to be far more responsible and thoughtful than their European predecessors. They moved with meticulous care to avoid the reductio ad absurdum of Europe. But, of course, the matter was different than having simply more thoughtful and sophisticated leaders. It was also that the United States and Soviet Union did not fear each other nearly as much as they feared war. That was not true of Germany, France, Russia and Britain. They might have been wrong not to appreciate the meaning of the machine gun, but they did not fear war more than their neighbors. And I can’t argue they were wrong. It had been a century since a systemic war, and no one alive knew what war could be like.

Are the horrors of the 31 years over? Germany is again united and an economic giant. The British are getting more uneasy with this. The French are not keeping up. And the Russians are returning to history. There is a new dinner party, and all the old guests have taken their seats. Fear of war is a deterrent, but men have feared war in the past and it has not deterred them. Having little fear of each other is a better deterrent. But with all of those at the table drinking toasts to their own wisdom, the Russian sitting at the end of the table, alone and bitter, is a reminder that what we intend, what we plan, and what will happen, are very different things.

It was only on the surface that the ministers or generals decided the outbreak of World War I. The real causes were the deep geopolitical and economic fears that trapped the decision makers in a way that rendered their intentions immaterial. It is deeply comforting, of course, to think that bad will and miscalculation created the war, because then the war could have been prevented if only men of good will and deeper understanding had been in charge. No one wants to think himself helpless—and by believing that World War I was the result of poor choices means that we are not helpless to prevent the terrible things that humans do.

In our own time, the monument to the belief in good intentions is, of course, the European Union. Bitter enemies, like France, Germany and Britain bound themselves together not only to prosper economically, but to make peace. There is little question but that the leaders of all of these nations crave peace and have no warlike intentions. Yet as Germany prospers, Spain and France sink into depression, and what used to be called Eastern Europe confronts rising Russian power. The deep divergences that the European Union was intended to bridge with understanding and bury under rational management are reemerging. History does not simply repeat itself, nor does it go away. If World War I teaches us anything, it is that the ability of the most rational of people to resist the forces dividing them can be overestimated.

The situation in Ukraine is illustrative. Ukraine is of fundamental importance to Russia for three reasons. First, their industrial and defense infrastructure is deeply interlocked. Second, Ukraine is a critical transit point for Russian energy going to Europe. And, third, it is a buffer against aggression from the European Peninsula. The immediate response might be that aggression from the European Peninsula is impossible, no one has the means or the intent. But the Russians never expected Napoleon’s invasion, and in 1932 Weimar Germany was quite friendly to Russia, as well as being economically and militarily crippled. Six years later, in 1938, Germany was both the leading military power in Europe and openly a threat to Russia.

Russia knows two things. The first is that the unexpected can happen. And, second, when it does—when the simply inconceivable does emerge—the catastrophe for Russia can be staggering. Nothing in Russian history gives it the right to assume the best. It is not hopes and promises and good intentions that protect Russia, but a stout borderland. In 1989, NATO was about a thousand miles from St. Petersburg. Today it is less than a hundred miles. With the potential integration of Ukraine into Western institutions, Russia would face a potential depth from territory near the terminus of the Nazi invasion.

Economic and energy issues aside, Russia cannot be indifferent to the growing power of the West, comforting itself with the idea that they mean no harm. At the same time, were Russia to return to a preeminent position in Ukraine, then Romania, Hungary, Slovakia and Poland could not simply assume Russian good will. Just as in World War I, it was capability and not intention that framed the actions of leaders, so today in Ukraine. Where the stakes are as they have been in the relationship between Russia and the European Peninsula, the right of Ukraine to determine its own fate falls by the wayside. Interdependence builds fear as interdependence deepens.

And, of course, in this not-quite-a-replay of history, the question increasingly being raised is the intention of Germany, united again, prosperous again, and aggressive again. It is not aggressive militarily. To the contrary, a military adventure is something Germany will try to avoid at all costs. Rather, it is aggressive in its exports. Germany exports the equivalent of 40 percent of its GDP, and it is as addicted to exports today as it was in 1900. And those exports are destabilizing European economies the same way they did over one hundred years ago. Russia supplies Germany with energy, and Germany has involved itself in the Ukraine.

World War I has taught us that nations do what they must, not what they will. Russia, France, Britain, and Germany are replaying an old game with different rules. But the same fears lurk beneath the surface. And the promise of Maastricht and the fall of the Soviet Union should not be taken from intentions. World War I teaches this frightening, yet reasonable, thought.